1. Field of the Invention
The present invention relates to a large part polishing apparatus and polishing method for polishing a large structural part such as a steam turbine part by projecting and colliding an abrasive particle having an elastic material as a core to the surface thereof.
2. Description of the Related Art
In steam turbines, in particular, moving vanes, stationary vanes, turbine rotors, and steam passage parts (for example, valve, steam tube, crossover tube, turbine inlet, turbine outlet, and nozzle box inside), their surface roughness has a key factor on the turbine performance, and it is required to improve the surface roughness (make smoother) of a steam passage as far as possible.
The turbine parts are manufactured in multiple processes, and in particular the manufacturing process for a stationary vane includes many processes such as a stationary vane cutting process, an assembling process, a welding process, a heat treatment process, a polishing process, a finish machining process, a final finishing process, and an inspection process. Therefore, the surface roughness is lowered and flaws are formed due to handling between processes, preparation works, and ambient environments. Besides since the shape is consists of many curved surfaces, it is hard to polish the entire surface perfectly.
A schematic configuration of a general steam turbine will be described by referring to FIGS. 11 to 14.
FIG. 11 is a bird's-eye view schematically showing an entire turbine rotor. As shown in the drawing, a turbine rotor 101 incorporates several stages to tens of stages of moving vanes 102 different in outside diameter in the axial direction, and each stage has tens to about a hundred moving vanes 102 planted in the peripheral direction of a rotor shaft 104, being supported by bearings 104a at both ends. The turbine rotor 101 is covered with a casing 103 forming a steam passage by these moving vanes and stationary vanes described below.
FIG. 12 is a detailed view showing a sectional view of one turbine unit in FIG. 11, in which a nozzle diaphragm 110 is disposed in the casing 103, and steam flows in the arrow direction of the figure in the steam passage formed by the moving vanes 102 planted in the turbine rotor 101.
FIG. 13A is a plan view of the nozzle diaphragm 110, and FIG. 13B is an outside view of the unit in the peripheral direction in FIG. 13A.
The stationary vane 105 has a curved portion 109 which forms a vane shape determined by the flow-in characteristic of steam flow as shown in FIG. 13B. The stationary vane 105 is enclosed by a diaphragm inner ring 106 and a diaphragm outer ring 107, and mechanically coupled by welding or the like. Reference numeral 108 is a seal fin fitting groove, and it is provided for installing a seal fin (not shown) for preventing steam from leaking out between the rotating turbine rotor 101 and the diaphragm inner ring 106 into the casing 103.
The surface roughness of the curved portion 109 of the stationary vane 105, diaphragm inner ring 106, and the stationary vane 105 side of the diaphragm outer ring 107 is known to be a key factor for influencing the turbine performance, in particular, as a result of experiment and running test of actual machines.
The number of stationary vanes 105 is determined by the turbine output characteristic (that is, steam flow rate) as clear from FIG. 13A, and the shape is very narrow and complicated because the vanes are provided in the nozzle diaphragm outer ring and inner ring at specific intervals.
On the other hand, the moving vanes 102 are not specifically shown in the sectional view, but have a distorted three-dimensional shape same as the stationary vanes. Since the vane shape is difference in each turbine stage, they are assembled after being polished through a manual polishing process by means of a power tool or air tool by using Bader machine, sand paper or the like. Depending on the turbine stage, since one moving vane may be longer than 1 meter, unexpected flaw, oxide scale or other defect may occur in the course of working, preparation, handling and transportation in and between processes.
Accordingly, to enhance the performance of the steam turbine, it is required to minimize surface roughness of turbine parts by polishing. However, since the turbine parts are very large in size, and extremely complicated in shape because of steam flow-in characteristic, and such narrow parts must be polished, it is hard to polish automatically or mechanically, and usually it has been attempted to improve the surface state of turbine parts by taking much time and labor by using compressed air and power rotary tools.
To assure the final surface roughness necessary to enhance the performance of turbine parts, finishing in a large shape as a nozzle diaphragm is needed, so that polishing work of very heavy duty, poor environment and long time is demanded.
On the other hand, in manual polishing tools, fine flaws may be formed when polishing narrow parts, in particular, it is hard to obtain a uniform polished surface, and the steam flow-in direction and orthogonal direction may not be polished uniformly, so that the polished surface tends to be uneven.
In air blasting or honing machine, the surface state becomes too rough, and therefore it is hard to achieve the surface roughness for achieving the turbine performance as stated above, and it is not suited as a final finishing tool.
Further, at the time of field checking or repair of an existing steam turbine, in order to obtain information accurately from nondestructive inspection, the surface is cleaned and oxide films are removed by air blasting using a ceramic projection material, but there is a room for further improvement from the viewpoint of enhancement of surface roughness.
Recently, an apparatus for polishing the surface of a member to be polished is proposed, in which an impeller is rotated to provide abrasive grains with centrifugal force, and the abrasive grains are sprayed to the member to be polished in a tangential direction around the impeller, thereby polishing or grinding the surface (Jpn. Pat. Appln. KOKAI Publication No. 11-347945, for example).